Pain. A typical "upset stomach" causes nausea and/or vomiting, and maybe a pain in the center of the stomach that might get better after vomiting. Appendicitis causes pain in the lower right side of the abdomen, often with loss of appetite, possibly a little diarrhea, and usually fever. The pain usually gets steadily worse. Often some tests are needed to see if it is an early/atypical appendicitis.